Then came Fred Hoyle who said 'Well, once you have carbon, you... that very eagerly captures another helium nucleus to make oxygen, so you get both carbon and oxygen, and from there on it's not difficult to get further.' The important thing was to... to bridge the gap from helium to carbon. Now how do you get the high temperature? This was I think found mostly by Schwarzschild at Princeton, I believe still a couple of years later, who... and I think I also should mention Icko Iben at Illinois who followed the evolution of a star, let's say from Sirius in his... its later life, and he found that such a star would exhaust its hydrogen, and when it exhausts its hydrogen then the center would collapse and the center would get very... very hot, and I think that was first found by Schwarzschild. And the... the center then gets hotter and hotter until you get the Salpeter reaction of helium getting together, making carbon, and then the temperature stabilizes at 1[00] or 200 million degrees and it stays there for a long, long time building up carbon and... and oxygen.

[Q] Right, just a small footnote: When Ed Salpeter does his calculation he points out, I believe, in the beryllium-8, the existence of an excited state which is important in making... And it's a similar idea that Hoyle points to, I mean, for the carbon-12. I mean, so it just, pointing out...

That's... that's the way it was.

[Q] Yea, and Salpeter's contribution is really very fundamental to this.

Yes.

[Q] I mean it's... and...

Well the general ideas of... proton-proton reaction, carbon nitrogen cycle, and then contraction to... to a very hot star, is generally accepted and... and one of the nice things is that some of the giant stars in their later evolution have near them clouds which are very rich in nitrogen. Now that is a direct observational confirmation of the carbon nitrogen cycle, namely in... in that cycle the most stable element is nitrogen. So while originally you may have made carbon and oxygen, once you assemble these in a star like Sirius, you manufacture mostly nitrogen, and when that star gets older you manufacture it closer to the surface and you have convection near the surface which pushes out some of that material and you then get a cloud near the brilliant star which... which emits the lines of nitrogen in fluorescence.

The late German-American physicist Hans Bethe once described himself as the H-bomb's midwife. He left Nazi Germany in 1933, after which he helped develop the first atomic bomb, won the Nobel Prize in Physics in 1967 for his contribution to the theory of nuclear reactions, advocated tighter controls over nuclear weapons and campaigned vigorously for the peaceful use of nuclear energy.

Silvan Sam Schweber is the Koret Professor of the History of Ideas and Professor of Physics at Brandeis University, and a Faculty Associate in the Department of the History of Science at Harvard University. He is the author of a history of the development of quantum electro mechanics, "QED and the men who made it", and has recently completed a biography of Hans Bethe and the history of nuclear weapons development, "In the Shadow of the Bomb: Oppenheimer, Bethe, and the Moral Responsibility of the Scientist" (Princeton University Press, 2000).